Good one. But as a non pilot old time flyer, just how high can today's jets sustain cruise in commercial conditions.
This is a limitation of the engines used. When you've got a big turbofan, it tends to give up at around 40,000ft simply because the air that bypasses the engine core is thinner and has less propulsive effects so there's less power available as you go higher. When you have something that is a pure jet where all the air flows through the core, such as the Olympus in Concorde, and others found in some fighter aircraft, they'll keep going much higher.